1. Field of the Invention
This invention relates to an optical waveguide element suitable for a second harmonic wave-generating element using e.g., a quasi-phase-matching.
2. Related Art Statement
For achieving a high density optical recording in an optical information processing technique, a blue laser to generate and oscillate a blue light of a 400 nm-430 nm wavelength at an output of 30 mW or more is desired, and thus, has been intensely researched and developed As a blue optical resource, an optical waveguide-type wavelength converting element, in which a laser to oscillate a red light as a fundamental wave and a second harmonic wave-generating element using a quasi-phase-matching are combined, has been expected.
For example, in xe2x80x9cElectronics Letters, Apr. 24, 1997, Vol. 33, No.9xe2x80x9d, pp806-807, it is described that a periodically polarized inversion structure is formed in a MgO-doped lithium niobate substrate, and an optical waveguide is formed perpendicularly to the periodically polarized structure by a proton-exchanging method to thereby fabricate an optical waveguide-type second harmonic wave-generating device.
Moreover, in xe2x80x9cTECHNICAL REPORT OF IEICE US95-24:EMD95-20:CPM95-32 (1995-07), pp31-38, it is described that a lithium niobate substrate is directly joined with a lithium tantalate substrate, and is ground and polished to be thinner to thereby fabricate a bulky optical waveguide from the thinned lithium niobate substrate. In this case, the surfaces of the lithium niobate substrate and the lithium tantalate substrate to be joined are flattened and cleaned, and hydrophilized. Then, hydroxyl-groups are absorbed on the surfaces of the substrates, and thermally treated to join the substrates. The hydroxyl-groups and hydrogen elements are desorbed gradually from the joined surfaces of the substrates, and thus, the substrates are strongly joined with each other to fabricate an optical waveguide device capable of confining a given optical wave. Furthermore, it is proposed that thus obtained optical waveguide device can be used for an optical waveguide device having a large optical damage-resistance, a large SHG effect, and an optical integrated circuit.
However, in the optical waveguide device shown in the above xe2x80x9cTECHNICAL REPORT OF IEICExe2x80x9d, the d constant (opto-electric constant) is likely to be degraded due to the difference in thermal expansion between the substrates. Therefore, a second harmonic wave-generating element having a periodically polarized conversion structure formed in the optical waveguide has only poor converting efficiency to a second harmonic wave. Particularly, the lithium niobate substrate and the lithium tantalate substrate are required to be directly joined at 100-1000xc2x0 C., more particularly 300xc2x0 C. or more. Therefore, the optical waveguide made of the thinned lithium niobate substrate is deformed due to the difference in thermal expansion between the substrates during the cooling down step after the joining step, resulting in the deterioration of the d constant and, in extreme cases, the mode change of the optical wave to be propagated. As a result, an optical waveguide device for practical use has not yet been provided.
It is an object of the present invention to prevent the deterioration of the d constant in such an optical waveguide element as mentioned above, which is made of a substrate and a bulky optical waveguide, while maintaining large optical damage-resistance characteristics.
For achieving the above object, this invention relates to an optical waveguide element including a substrate made of a lithium tantalate single crystal or a lithium niobate-lithium tantalate solid solution single crystal and a bulky optical waveguide made of a lithium niobate single crystal directly joined to the substrate. The c-axis of the single crystal constituting the substrate is tilted with respect to the surface of the substrate to be joined.
According to the present invention, the offset substrate and the bulky optical waveguide made of the lithium niobate single crystal are employed. Moreover, the offset substrate and the bulky optical waveguide are directly joined. Therefore, the bulky optical waveguide can keep the good crystallinity, and thus, the optical waveguide element can have lower optical propagation-loss. At the same time, the thermal expansions of the offset substrate and the bulky optical waveguide can be matched two-dimensionally in the whole range of the joined surface thereof. As a result, the d constant of the thus obtained optical waveguide element according to the present invention can be developed while maintaining the lower optical damage-resistance.
Particularly, a second harmonic wave-generating device for practical use, having a lower optical damage in the optical waveguide and a stable d constant, is fabricated using the above-mentioned optical waveguide element. Therefore, the present invention can greatly contribute to industrial applications.
In the case of fabricating a higher harmonic wave-generating device, particularly a second harmonic wave-generating device, a higher harmonic wave of 330-550 mm, particularly 400-430 nm can be obtained.